Humidity 60%, temperature 24°C, EC and pH within range — and the tomato in week three is showing upward-curling leaf margins and signs of calcium deficiency. The agronomist says "add more calcium," you add it — and it doesn't help. The problem is not calcium — it's transpiration. The plant is not moving enough water to deliver calcium to the leaf margins, and no dose of calcium in the reservoir will fix that until the conditions change.
Quick glossary: VPD (Vapor Pressure Deficit) — the difference between the maximum possible vapour pressure at a given temperature and the actual vapour pressure in the air; it shows the "dryness" of the air from the plant's perspective and determines how strongly the air "pulls" water from the leaf. Transpiration — evaporation of water through the stomata of the leaf; the driving force that moves water and dissolved elements from root to leaf. Stomata — microscopic openings on the leaf through which gas exchange and transpiration occur; at low VPD the plant closes its stomata to avoid losing too much water.
What VPD Measures and Why RH Alone Is Not Enough
Relative humidity 60% at 20°C and 60% at 30°C — the same number, but fundamentally different states for the plant. At 30°C the air can hold twice as much water vapour as at 20°C — and the same 60% RH means a much greater "pull" on the plant at higher temperature.
VPD measures this pull directly — in kilopascals (kPa). The higher the VPD, the more strongly the air draws water from the leaf, and the more active the transpiration. The lower the VPD, the less the air "pulls" — and the less actively the plant moves water and elements from root to leaf.
VPD is straightforward to calculate: first determine the saturation vapour pressure (SVP) at leaf temperature (approximately equal to air temperature), then subtract the actual vapour pressure (RH × SVP). There are ready-made calculators and tables where entering temperature and RH is sufficient.
What Happens at Low and High VPD
Low VPD (below 0.4–0.5 kPa) — the air is nearly saturated with moisture, the pull on the plant is minimal. Transpiration slows, stomata close. Calcium and magnesium, which move passively with the water flow, reach the leaves more slowly. Typical symptoms: tip burn in lettuce and greens, calcium deficiency in upper leaves and tomato fruits (blossom-end rot) even with correct EC. Additionally — at low VPD there is virtually no driving force for evaporation from the leaf surface and moisture droplets persist longer — ideal conditions for fungi and bacteria.
High VPD (above 1.5–2.0 kPa) — the air is very dry relative to the plant. Transpiration is excessive; the plant may not be able to supply water from the root as fast as it evaporates. Stomata close as a protective response — and with closed stomata not only does transpiration stop, but CO₂ uptake stops too, meaning photosynthesis halts. Typical symptoms: wilting shoot tips with a moist substrate, leaf margin burn, slowed growth rate even with adequate lighting.
Working VPD Ranges by Growth Phase
Different growth phases require different VPD — and this matters more than a single "correct" range:
Germination and early vegetative: 0.4–0.8 kPa — a young plant has a weak root system and cannot compensate for excessive transpiration.
Active vegetative: 0.8–1.2 kPa — the optimum for most crops; a balance between active water flow and avoiding excessive stress.
Fruiting and ripening: 1.0–1.5 kPa — higher VPD stimulates greater water flow and more intense fruit fill.
These ranges apply to leaf temperature under normal lighting. At night without lighting and transpiration, the requirement is different — avoiding condensation matters more than hitting a specific VPD.
How to Manage VPD in the Greenhouse
VPD is determined by two parameters: temperature and humidity. Changing VPD means changing one or both.
Raise VPD (if low, transpiration is weak): increase temperature or reduce humidity. Lower humidity — open ventilation or run a dehumidifier. Raise temperature — turn on heating or increase lighting intensity (lamps generate heat).
Lower VPD (if high, the plant is overheating): reduce temperature or raise humidity. Humidifiers, fogging, reducing lighting intensity during heat.
The connection with temperature and humidity is direct: managing VPD is managing temperature and humidity together, not separately. When changing one parameter, always calculate what happens to VPD.
Three Mistakes That Cost the Most
Treating calcium deficiency without checking VPD. Tip burn and blossom-end rot are classic symptoms of low VPD where calcium simply is not being delivered to the endpoints. Raising calcium in the solution does not help in this case. The first step is to measure VPD and confirm it is within the working range.
Setting the same VPD for all growth phases. 1.2 kPa — optimal for fruiting — is too high for seedlings with a weak root system. Adapt conditions to the phase and don't hesitate to change the target VPD when transitioning between phases.
Measuring RH and temperature but not calculating VPD. "Humidity 65%, temperature 26°C — looks fine" — but VPD at that point is already 1.4 kPa, the upper threshold for active vegetative growth. Without calculating VPD, it is impossible to know where conditions actually stand even with "normal" individual readings. Free online calculators and tables are available — use them daily.
How to Know VPD Is Within Normal Range
VPD is calculated (not "looks fine" from individual readings) and sits within the range appropriate for the growth phase. Plants are transpiring actively — guttation droplets visible at leaf tips in the morning during vegetative phase. No calcium deficiencies at growth points with correct EC. Stomata are open — the plant is growing actively and responding to CO₂.
For deeper understanding: Temperature and Humidity in the Greenhouse: Norms, Interaction, and Common Mistakes — explains how VPD is formed from temperature and humidity together and how to manage both parameters so that VPD stays within the working range throughout the day.